1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly, it relates to an AlGaInP-based high-output red semiconductor laser device.
2. Description of the Prior Art
An AlGaInP-based high-output red semiconductor laser device is generally known as a semiconductor laser device applicable to a recordable DVD system or the like. In order to improve the recording speed in the recordable DVD system, the intensity of a laser beam applied to the disk must be improved. In order to improve the intensity of the laser beam applied to the disk, the coupling efficiency between an objective lens for focusing the beam on the disk and the laser beam must be improved while the output of the semiconductor laser device as the light source must be increased.
Improvement of the coupling efficiency between the objective lens and the laser beam is now studied. In general, the objective lens is provided in response to an angle of horizontal divergence of the laser beam, and hence a vertical laser beam component having a larger angle than the horizontal divergence angle is applied outward beyond the objective lens. In this case, the coupling efficiency between the objective lens and the laser beam is reduced. In order to improve the coupling efficiency between the objective lens and the laser beam, therefore, the vertical beam divergence angle must be reduced. In other words, the ratio of the vertical beam divergence angle to the horizontal beam divergence angle (aspect ratio: vertical beam divergence angle/horizontal beam divergence angle) must be approached to 1.0 with respect to an active layer of the semiconductor laser device.
In order to increase the output of the semiconductor laser device, the level of COD (catastrophic optical damage: deterioration of an end face emitting the laser beam) must indispensably be improved. It is known that COD is caused in the following cycle: When a current is injected in a laser end face having surface states in high density, non-radiative recombination results through the surface states. Therefore, the laser end face generates heat. An energy gap is reduced in the active layer on the laser end face due to this heat, to increase light absorption. Thus, heat is further generated. The temperature of the laser end face is increased due to this cycle, whereby crystals are fused to break the laser end face as a result.
A method employing a window structure by Zn diffusion is generally known as a method of suppressing such COD. This method is disclosed in IEEE Journal of Quantum Electronics, Vol. 29, No. 6, pp. 1874 to 1877 (1993), for example. In the conventional method employing a window structure, an impurity is introduced into a region of an active layer of a laser device close to a cavity end face thereby disordering a quantum well structure of the active layer. Therefore, a band gap of the region of the active layer close to the cavity end face is spread as compared with the remaining regions, thereby reducing light absorption on the cavity end face. Thus, temperature increase can be suppressed on the laser end face, thereby suppressing COD.
A method of reducing optical density on a portion of an active layer close to a cavity end face by enlarging the area of an emission spot is also known as another method of suppressing COD. In this case, the area of the emission part is so enlarged as to reduce the vertical beam divergence angle.
When the vertical beam divergence angle is reduced, a kink (bend of a current-light output characteristic) originating from higher transverse mode hardly results. When the vertical beam divergence angle is reduced, therefore, not only suppression of the aforementioned COD but also improvement of the light output can be attained.
In general, the output of the semiconductor laser must be increased along with improvement of the coupling efficiency between the objective lens and the laser beam in order to improve the intensity of the laser beam applied to the disk, as hereinabove described. In order to improve the coupling efficiency between the objective lens and the laser beam, the aspect ratio, i.e., the ratio of the vertical beam divergence angle to the horizontal beam divergence angle, must be reduced. In order to increase the output of the semiconductor laser device, the COD level or the kink level must be improved. It is generally known that the aspect ratio can be reduced, and the COD and kink levels can be improved by reducing the vertical beam divergence angle. In order to reduce the vertical beam divergence angle, the emission spot may be enlarged.
When the emission spot is enlarged in a conventional semiconductor laser device having a loss guided structure confining transverse light by absorbing light in a current blocking layer in order to reduce the vertical beam divergence angle, however, light absorption in the current blocking layer is increased to reduce inclination (slope efficiency) of the current-light output characteristic. Therefore, an operating current for obtaining a constant light output is disadvantageously increased. When the operating current is increased, light output is readily saturated (light output thermal saturation) by heat generation, and hence it is difficult to improve the light output. Thus, it is difficult to increase the output by reducing the vertical beam divergence angle in the conventional semiconductor laser device having a loss guided structure.
To this end, generally known is a method of reducing light absorption in a current blocking layer through a real refractive index guided structure transparentizing the current blocking layer with respect to a laser beam. According to this real refractive index guided structure, a light confinement layer consisting of a material having a smaller refractive index than a cladding layer is provided to cover the side surfaces of the cladding layer of a ridge portion thereby confining transverse light due to difference between refractive indices. Thus, the current blocking layer absorbs no light, to hardly cause light output thermal saturation resulting from light absorption in the current blocking layer.
When the area of an emission spot is enlarged for reducing the vertical beam divergence angle in a conventional semiconductor laser device having such a real refractive index guided structure, however, the ratio of the light component (optical confinement factor) present in the portion of the active layer to the overall area of the light is reduced. Therefore, the light hardly attains a gain, leading to difficulty in lasing. It is known that a threshold current is thus increased to increase an operating current, readily leading to light output thermal saturation.
Thus, when the emission spot is enlarged for reducing the vertical beam convergence angle in the conventional laser device having a real refractive index guided structure, light output thermal saturation readily results from reduction of the optical confinement factor although light output thermal saturation hardly results from light absorption in the current blocking layer, and hence it has been regarded as difficult to improve a kink light output and obtain a high maximum light output. Consequently, there has been developed no red laser device of a real refractive index guided structure having a low aspect ratio with a vertical beam divergence angle of not more than 20.0xc2x0.
An object of the present invention is to provide a semiconductor laser device having a real refractive index guided structure capable of obtaining a high kink light output and a high maximum light output also when reducing a vertical beam divergence angle.
Another object of the present invention is to implement a low aspect ratio in the aforementioned semiconductor laser device.
Noting the aforementioned point, the inventors have made various experiments and deep study, to find out that a high kink light output and a high maximum light output can be obtained in a semiconductor laser device having a real refractive index guided structure also when a vertical beam divergence angle is set to a small level of at least 12.5xc2x0 and not more than 20.0xc2x0. The present invention is now described.
According to a first aspect of the present invention, a semiconductor laser device having a real refractive index guided structure comprises an n-type cladding layer of AlGaInP formed on an n-type GaAs substrate, an active layer having an AlGaInP layer formed on the n-type cladding layer, a p-type cladding layer of AlGaInP formed on the active layer and a light confinement layer formed to partially cover the p-type cladding layer, and a vertical beam divergence angle is at least 12.5xc2x0 and not more than 20.0xc2x0.
According to the first aspect, the real refractive index guided semiconductor laser device is so structured that the vertical beam divergence angle is at least 12.5xc2x0 and not more than 20.0xc2x0 as hereinabove described, whereby a higher kink light output and a higher maximum light output can be obtained as compared with a conventional semiconductor laser beam device having a vertical beam divergence angle exceeding 20.0xc2x0.
In the aforementioned semiconductor laser device according to the first aspect, the vertical beam divergence angle is preferably an angle exhibiting thermal saturation before a laser beam emitting end face is deteriorated. When the vertical beam divergence angle is set to such an angle, the laser beam emitting end face can be prevented from deterioration (COD). In this case, the vertical beam divergence angle is preferably smaller than 18xc2x0. More preferably, the vertical beam divergence angle is at least 12.5xc2x0 and not more than 17.0xc2x0 in this case. This range of at least 12.5xc2x0 and not more than 17.0xc2x0 has been experimentally confirmed as a range causing no deterioration of the laser beam emitting end face (COD) in practice, and hence the laser beam emitting end face can be reliably prevented from deterioration (COD) when the vertical beam divergence angle is set within this range.
In the aforementioned case, the vertical beam divergence angle is preferably at least 15.5xc2x0. At this lower limit angle of 15.5xc2x0, a light output substantially identical to that at an angle of 18.0xc2x0 causing deterioration (COD) of the laser beam emitting end face with the maximum light output can be obtained with neither deterioration (COD) of the laser beam emitting end face nor kink, and hence a higher light output can be obtained by setting the vertical beam divergence angle to at least 15.5xc2x0.
In the aforementioned semiconductor laser device according to the first aspect, the n-type cladding layer or the p-type cladding layer having a smaller thickness is preferably at least 1.5 xcexcm and not more than 2.5 xcexcm in thickness. According to this structure, a semiconductor laser device having a vertical beam divergence angle of at least 12.5xc2x0 and not more than 15.0xc2x0 can be readily obtained.
In the aforementioned semiconductor laser device according to the first aspect, the active layer preferably includes a light guide layer and a barrier layer, for adjusting the vertical beam divergence angle by changing at least either the Al composition of the light guide layer and the barrier layer or the thickness of the light guide layer. According to this structure, the vertical beam divergence angle can be readily adjusted without changing the degree of carrier confinement in the active layer and without remarkably changing the lasing wavelength.
In the aforementioned semiconductor laser device according to the first aspect, the active layer preferably includes a light guide layer and a barrier layer, the composition ratio Al/(Al+Ga) of the light guide layer and the barrier layer is preferably at least 0.39 and not more than 0.67, and the thickness of the light guide layer is preferably at least 15 nm and not more than 25 nm. According to this structure, a semiconductor laser device having a vertical beam divergence angle of at least 12.5xc2x0 and not more than 20.0xc2x0 can be readily obtained.
In the aforementioned semiconductor laser device according to the first aspect, the active layer preferably has a quantum well structure, and an impurity is preferably introduced into a region of the active layer close to a cavity end face thereby disordering the quantum well structure and widening a band gap as compared with the remaining regions. According to this structure, absorption of a laser beam is suppressed in the vicinity of the end face, whereby the end face can be inhibited from generating heat. Thus, the laser beam emitting end face can be effectively prevented from deterioration (COD), whereby a high maximum light output can be obtained.
The aforementioned semiconductor laser device according to the first aspect preferably has a maximum light output of at least 150 mW. According to this structure, a semiconductor laser device having a higher maximum light output than the conventional device can be obtained.
According to a second aspect of the present invention, a semiconductor laser device having a real refractive index guided structure comprises an n-type cladding layer of AlGaInP formed on an n-type GaAs substrate, an active layer having an AlGaInP layer formed on the n-type cladding layer, a p-type cladding layer of AlGaInP formed on the active layer and a light confinement layer formed to partially cover the p-type cladding layer, while the n-type cladding layer or the p-type cladding layer having a smaller thickness is at least 1.5 xcexcm and not more than 2.5 xcexcm in thickness, and a vertical beam divergence angle is at least 12.5xc2x0 and not more than 15.0xc2x0.
According to the second aspect, the real refractive index guided semiconductor laser device is so structured that the vertical beam divergence angle is at least 12.5xc2x0 and not more than 15.0xc2x0 as hereinabove described, whereby a higher maximum light output can be obtained as compared with a conventional semiconductor laser device having a vertical beam divergence angle exceeding 20.0xc2x0 and a low aspect ratio can be implemented.
According to a third aspect of the present invention, a semiconductor laser device having a real refractive index guided structure comprises an n-type cladding layer of AlGaInP formed on an n-type GaAs substrate, an active layer having an AlGaInP layer formed on the n-type cladding layer, a p-type cladding layer of AlGaInP formed on the active layer and a light confinement layer formed to partially cover the p-type cladding layer, and a vertical beam divergence angle is at least 15.0xc2x0 and not more than 20.0xc2x0.
According to the third aspect, the real refractive index guided semiconductor laser device is so structured that the vertical beam divergence angle is at least 15.0xc2x0 and not more than 20.0xc2x0 as hereinabove described, whereby a higher kink light output and a higher maximum light output can be obtained as compared with the conventional semiconductor laser device having a vertical beam divergence angle exceeding 20.0xc2x0.
In the aforementioned semiconductor laser device according to the third aspect, the vertical beam divergence angle is preferably smaller than 18xc2x0. When the vertical beam divergence angle is set to such a level, a laser beam emitting end face can be prevented from deterioration (COD). In this case, the vertical beam divergence angle is more preferably at least 15.0xc2x0 and not more than 17.0xc2x0. This range of at least 15.0xc2x0 and not more than 17.0xc2x0 has been confirmed as causing no deterioration (COD) of the laser beam emitting end face in practice, and hence the laser beam emitting end face can be reliably prevented from deterioration (COD) when the vertical beam divergence angle is set within this range.
In the aforementioned semiconductor laser device according to the third aspect, the active layer preferably has a quantum well structure, and an impurity is preferably introduced into a region of the active layer close to a cavity end face thereby disordering the quantum well structure and widening a band gap as compared with the remaining regions. According to this structure, absorption of a laser beam is suppressed in the vicinity of the end face, whereby the end face can be inhibited from generating heat. Thus, the laser beam emitting end face can be effectively prevented from deterioration (COD), whereby a high maximum light output can be obtained.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.